Francisella tularensis (Ft), a Gram negative intracellular bacterium, the etiologic agent of tularemia, and is classified as a Category A agent because it can contracted with low inocula by the respiratory route and causes rapid morbidity and mortality if untreated. While attenuated for humans, mice infected i.n. or i.p. with Ft Live Vaccine Strain (LVS), a type B strain, contract a tularemia-like disease. In this grant cycle, we reported that Ft LVS induces in mice a potent inflammatory response in vivo and in vitro. Using Ft LVS mutants that fail to escape from the phagosome or fail to replicate intracellularly in murine macrophages, we identified the signaling pathways by the host innate immune system is activated. Ft LVS induces TLR2- mediated signaling; however, once Ft escapes from the phagosome into the cytosol, IFN-J3 is produced and reutilized, and the inflammasome activated. Despite this robust inflammatory response, the host succumbs to infection. Our data indicate that Ft LVS initiates a strong inflammatory response leading to development of classically activated macrophages (CA-M0); however, once IL-4 and IL-13 are induced by infected macrophages, they differentiate into alternatively activated (AA-M0), thereby facilitating intracellular replication. Ft LVS-infected IL-4Ra~'~ and STAT6' macrophages fail to differentiate into AA-M0 and control intracellular replication, while Ft LVS-infected macrophages treated with rIFN-p restrict bacterial growth. Ft LVS lipopolysaccharide (LPS), a poor TLR4 agonist, fails to induce an inflammatory response. However, when mice are vaccinated with Ft LVS LPS 2 days prior to lethal i.p. challenge, a protective anti-LPS antibody response is induced by splenic B-1a cells. We will now seek novel ways to control respiratory infection with Ft LVS and Schu S4, a virulent type A strain, by testing the hypotheses that (1) Schu S4, like Ft LVS, drives AA-M0 to evade host responses; (2) agents that interfere with development ofAA-M0 will increase the anti-microbial response in vivo; and, (3) increasing the immunogenicity of a Ft LVS LPS vaccine will protect mice against respiratory and systemic challenge with Ft LVS and Schu S4. The main objective of this proposal is to develop novel therapies for tularemia based on interfering with development of AA-M0, as well as improve development of a safe vaccine(s) that protects against both type A and B strains.